RNA labeling method

ABSTRACT

Methods are described in which a sample containing RNA is contacted with an enzyme having an RNA ligation activity in the presence of a labeled substrate to provide labeled RNA. Methods of performing an array analysis of a labeled RNA sample are also described.

FIELD OF THE INVENTION

The invention relates generally to methods of biochemical analysis. More specifically, the invention relates to providing a method of attaching an observable label to RNA.

BACKGROUND OF THE INVENTION

Straightforward and reliable methods for simultaneously analyzing several constituents of a complex sample are extremely desirable. Polynucleotide arrays (such as DNA or RNA arrays) are known and are used, for example, as diagnostic or screening tools. Such arrays include regions of usually different sequence polynucleotides (“capture agents”) arranged in a predetermined configuration on a support. The arrays are “addressable” in that these regions (sometimes referenced as “array features”) have different predetermined locations (“addresses”) on the support of array. The polynucleotide arrays typically are fabricated on planar supports either by depositing previously obtained polynucleotides onto the support in a site specific fashion or by site specific in situ synthesis of the polynucleotides upon the support. After depositing the polynucleotide capture agents onto the support, the support is typically processed (e.g., washed and blocked for example) and stored prior to use.

In use, an array is contacted with a sample or labeled sample containing analytes (typically, but not necessarily, other polynucleotides) under conditions that promote specific binding of the analytes in the sample to one or more of the capture agents present on the array. Thus, the arrays, when exposed to a sample, will undergo a binding reaction with the sample and exhibit an observed binding pattern. This binding pattern can be detected upon interrogating the array. For example all target polynucleotides (for example, DNA) in the sample can be labeled with a suitable label (such as a fluorescent compound), and the label then can be accurately observed (such as by observing the fluorescence pattern) on the array after exposure of the array to the sample. Assuming that the different sequence polynucleotides were correctly deposited in accordance with the predetermined configuration, then the observed binding pattern will be indicative of the presence and/or concentration of one or more components of the sample. Techniques for scanning arrays are described, for example, in U.S. Pat. No. 5,763,870 and U.S. Pat. No. 5,945,679. Still other techniques useful for observing an array are described in U.S. Pat. No. 5,721,435.

There has been great interest in the analysis of small RNAs, such as short interfering RNAs (siRNAs), microRNAs (miRNA), tiny non-codingRNAs (tncRNA) and small modulatory RNA (smRNA), since the discovery of siRNA biological activity over a decade ago. See Novina et al., Nature 430: 161-164 (2004). Even though the functions of most discovered miRNAs remain a mystery, it has become clear that they exist in abundance in plants and animals, with up to tens of thousands of copies per cell. In the fruit fly, 78 have been identified, and over 200 have been identified in human (see the public database accessible via the website located at >>http://www.sanger.ac.uk/cgi-bin/Rfam/mirna/browse.pl<<). The levels of individual miRNAs seem to vary with developmental stages and tissue types. The level of fluctuation may be correlated with phenotype, mRNA levels, or protein levels for better biological insight. Thus quantitative measurements of miRNA may be of great importance. Further, viral miRNAs have been identified and may play a role in latency (see Pfeffer et al., Science, 304: 734-736 (2004)), making the detection and quantification of miRNAs a potentially valuable diagnostic tool.

Analytic methods employing polynucleotide arrays have been used for investigating these small RNAs, e.g. miRNAs have become a subject of investigation with microarray analysis. See, e.g., Liu et al., Proc. Nat'l Acad. Sci. USA, 101: 9740-9744 (2004); Thomson et al., Nature Methods, 1: 1-7 (2004); and Babak et al., RNA, 10: 1813-1819 (2004). Methods of labeling RNAs are of interest for use in array analysis of RNA to provide an observable label used in interrogating the array. In the study of Liu et al., the miRNA was transcribed into DNA with a biotin-labeled primer. This primer was subsequently labeled with streptavidin-linked Alexa dye prior to array hybridization. This method is susceptible to any reverse-transcriptase reaction bias. Further, the streptavidin-dye as well as streptavidin-biotin-RNA stochiometry may be difficult to quantify. In the study of Thomson et al., the miRNA was directly labeled with 5′-phosphate-cytidyl-uridyl-Cy3-3′ using T4 RNA ligase. This reaction is sensitive to the acceptor sequence. See England et al., Biochemistry, 17: 2069-2776 (1978). In the study of Babak et al (4), the miRNA was labeled with Ulysis Alexa Fluor system, which reacts with guanine residue (G) of RNA. Since different miRNAs do not have uniform G content, this method is not quantitative.

Thus, there is a continuing need for methods of labeling RNA with an observable label. Such methods may be used in conjunction with analytical methods based on observing the label, such as array-based analysis of polynucleotides.

SUMMARY OF THE INVENTION

The invention thus relates to novel methods for labeling RNA in a sample. In particular embodiments, the invention provides methods in which a sample containing RNA is contacted with an enzyme having an RNA ligation activity in the presence of a labeled substrate. This is done under conditions sufficient to result in coupling of the labeled substrate to the RNA in the sample to provide labeled RNA, the conditions including a DMSO concentration in the range from about 20% to about 30%. The labeled substrate includes an observable label moiety attached to a nucleotide moiety.

Methods of performing an array analysis of an RNA sample are also taught herein. In certain embodiments, the invention provides a method of performing an array analysis wherein the method includes labeling the RNA in the sample to provide labeled RNA. The labeled RNA is then contacted with an array under conditions sufficient to provide for specific binding of labeled RNA to the array. The array typically is then interrogated to provide data on binding of RNA in the sample to the array.

Additional objects, advantages, and novel features of this invention shall be set forth in part in the descriptions and examples that follow and in part will become apparent to those skilled in the art upon examination of the following specifications or may be learned by the practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instruments, combinations, compositions and methods particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the invention will be understood from the description of representative embodiments of the method herein and the disclosure of illustrative apparatus for carrying out the method, taken together with the Figures, wherein

FIG. 1 schematically illustrates embodiments of the present invention.

FIG. 2 schematically illustrates other embodiments of the present invention.

To facilitate understanding, identical reference numerals have been used, where practical, to designate corresponding elements that are common to the Figures. Figure components are not drawn to scale.

DETAILED DESCRIPTION

Before the invention is described in detail, it is to be understood that unless otherwise indicated this invention is not limited to particular materials, reagents, reaction materials, manufacturing processes, or the like, as such may vary. It is also to be understood that the terminology used herein is for purposes of describing particular embodiments only, and is not intended to be limiting. It is also possible in the present invention that steps may be executed in different sequence where this is logically possible. However, the sequence described below is preferred.

It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “an insoluble support” includes a plurality of insoluble supports. Similarly, reference to “an RNA” includes a plurality of different identity (sequence) RNA species.

Furthermore, where a range of values is provided, it is understood that every intervening value, between the upper and lower limit of that range and any other stated or intervening value in that stated range is encompassed within the invention. Also, it is contemplated that any optional feature of the inventive variations described may be set forth and claimed independently, or in combination with any one or more of the features described herein. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for use of such exclusive terminology as “solely,” “only,” and the like in connection with the recitation of claim elements, or use of a “negative” limitation. In this specification and in the claims that follow, reference will be made to a number of terms that shall be defined to have the following meanings unless a contrary intention is apparent.

“Optional” or “optionally” means that the subsequently described circumstance may or may not occur, so that the description includes instances where the circumstance occurs and instances where it does not. For example, if a step of a process is optional, it means that the step may or may not be performed, and, thus, the description includes embodiments wherein the step is performed and embodiments wherein the step is not performed (i.e. it is omitted).

An “oligonucleotide” is a molecule containing from 2 to about 100 nucleotide subunits. The term “nucleic acid” and “polynucleotide” are used interchangeably herein to describe a polymer of any length composed of nucleotides, e.g., deoxyribonucleotides or ribonucleotides, or compounds produced synthetically (e.g., PNA as described in U.S. Pat. No. 5,948,902 and the references cited therein) which can hybridize with naturally occurring nucleic acids in a sequence specific manner analogous to that of two naturally occurring nucleic acids, e.g., can participate in Watson-Crick base pairing interactions. The terms “nucleoside” and “nucleotide” are intended to include those moieties that contain not only the known purine and pyrimidine bases, but also other heterocyclic bases that have been modified. Such modifications include methylated purines or pyrimidines, acylated purines or pyrimidines, alkylated riboses or other heterocycles. In addition, the terms “nucleoside” and “nucleotide” include those moieties that contain not only conventional ribose and deoxyribose sugars, but other sugars as well. Modified nucleosides or nucleotides also include modifications on the sugar moiety, e.g., wherein one or more of the hydroxyl groups are replaced with halogen atoms or aliphatic groups, or are functionalized as ethers, amines, or the like. “Analogues” refer to molecules having structural features that are recognized in the literature as being mimetics, derivatives, having analogous structures, or other like terms, and include, for example, polynucleotides incorporating non-natural (not usually occurring in nature) nucleotides, unnatural nucleotide mimetics such as 2′-modified nucleosides, peptide nucleic acids, oligomeric nucleoside phosphonates, and any polynucleotide that has added substituent groups, such as protecting groups or linking moieties.

“Moiety” and “group” are used to refer to a portion of a molecule, typically having a particular functional or structural feature, e.g. a linking group (a portion of a molecule connecting two other portions of the molecule), or an ethyl moiety (a portion of a molecule with a structure closely related to ethane). A moiety is generally bound to one or more other moieties to provide a molecular entity. As a simple example, a hydroxyl moiety bound to an ethyl moiety provides an ethanol molecule. At various points herein, the text may refer to a moiety by the name of the most closely related structure (e.g. an oligonucleotide moiety may be referenced as an oligonucleotide, a mononucleotide moiety may be referenced as a mononucleotide). However, despite this seeming informality of terminology, the appropriate meaning will be clear to those of ordinary skill in the art given the context, e.g. if the referenced term has a portion of its structure replaced with another group, then the referenced term is usually understood to be the moiety. For example, a mononucleotide moiety is a single nucleotide which has a portion of its structure (e.g. a hydrogen atom, hydroxyl group, or other group) replaced by a different moiety (e.g. a linking group, an observable label moiety, or other group). Similarly, an oligonucleotide moiety is an oligonucleotide which has a portion of its structure (e.g. a hydrogen atom, hydroxyl group, or other group) replaced by a different moiety (e.g. a linking group, an observable label moiety, or other group). “Nucleotide moiety” is generic to both mononucleotide moiety and oligonucleotide moiety.

“Linkage” as used herein refers to a first moiety bonded to two other moieties, wherein the two other moieties are linked via the first moiety. Typical linkages include ether (—O—), oxo (—C(O)—), amino (—NH—), amido (—N—C(O)—), thio (—S—), phospho (—P—), ester (—O—C(O)—).

“Bound” may be used herein to indicate direct or indirect attachment. In the context of chemical structures, “bound” (or “bonded”) may refer to the existence of a chemical bond directly joining two moieties or indirectly joining two moieties (e.g. via a linking group or any other intervening portion of the molecule). The chemical bond may be a covalent bond, an ionic bond, a coordination complex, hydrogen bonding, van der Waals interactions, or hydrophobic stacking, or may exhibit characteristics of multiple types of chemical bonds. In certain instances, “bound” includes embodiments where the attachment is direct and also embodiments where the attachment is indirect. “Free,” as used in the context of a moiety that is free, indicates that the moiety is available to react with or be contacted by other components of the solution in which the moiety is a part.

“Isolated” or “purified” generally refers to isolation of a substance (compound, polynucleotide, protein, polypeptide, polypeptide, chromosome, etc.) such that the substance comprises a substantial portion of the sample in which it resides (excluding solvents), i.e. greater than the substance is typically found in its natural or un-isolated state. Typically, a substantial portion of the sample comprises at least about 5%, at least about 10%, at least about 20%, at least about 30%, at least about 50%, preferably at least about 80%, or more preferably at least about 90% of the sample (excluding solvents). For example, a sample of isolated RNA will typically comprise at least about 5% total RNA, where percent is calculated in this context as mass (e.g. in micrograms) of total RNA in the sample divided by mass (e.g. in micrograms) of the sum of (total RNA+other constituents in the sample (excluding solvent). Techniques for purifying polynucleotides and polypeptides of interest are well known in the art and include, for example, gel electrophresis, ion-exchange chromatography, affinity chromatography, flow sorting, and sedimentation according to density. In typical embodiments, one or more of the sample, the enzyme having an RNA ligation activity, and the labeled substrate is in isolated form; more typically, all three are obtained in isolated form prior to use in the present methods.

The term “sample” as used herein relates to a material or mixture of materials, typically, although not necessarily, in fluid form, containing one or more components of interest.

The term “analyte” is used herein to refer to a known or unknown component of a sample. In certain embodiments of the invention, an analyte may specifically bind to a capture agent on a support surface if the analyte and the capture agent are members of a specific binding pair. In general, analytes are typically RNA or other polynucleotides. Typically, an “analyte” is referenced as a species in a mobile phase (e.g., fluid), to be detected by a “capture agent” which, in some embodiments, is bound to a support, or in other embodiments, is in solution. However, either of the “analyte” or “capture agent” may be the one which is to be evaluated by the other (thus, either one could be an unknown mixture of components of a sample, e.g., polynucleotides, to be evaluated by binding with the other). A “target” references an analyte.

The term “capture agent” refers to an agent that binds an analyte through an interaction that is sufficient to permit the agent to bind and concentrate the analyte from a homogeneous mixture of different analytes. The binding interaction may be mediated by an affinity region of the capture agent. Representative capture agents include polypeptides and polynucleotides, for example antibodies, peptides, or fragments of double stranded or single-stranded DNA or RNA may employed. Capture agents usually “specifically bind” one or more analytes.

The term “specific binding” refers to the ability of a capture agent to preferentially bind to a particular analyte that is present in a homogeneous mixture of different analytes. In certain embodiments, a specific binding interaction will discriminate between desirable and undesirable analytes in a sample, in some embodiments more than about 10 to 100-fold or more (e.g., more than about 1000- or 10,000-fold). In certain embodiments, the binding constant of a capture agent and analyte is greater than 10⁶ M⁻¹, greater than 10⁷ M⁻¹, greater than 10⁸ M⁻¹, greater than 10⁹ M⁻¹, greater than 10¹⁰ M⁻¹, usually up to about 10¹² M⁻¹, or even up to about 10¹⁵ M⁻¹.

The term “stringent assay conditions” as used herein refers to conditions that are compatible to produce binding pairs of nucleic acids, e.g., capture agents and analytes, of sufficient complementarity to provide for the desired level of specificity in the assay while being incompatible to the formation of binding pairs between binding members of insufficient complementarity to provide for the desired specificity. Stringent assay conditions are the summation or combination (totality) of both hybridization and wash conditions.

A “stringent hybridization” and “stringent hybridization wash conditions” in the context of nucleic acid hybridization (e.g., as in array, Southern or Northern hybridizations) are sequence dependent, and are different under different experimental conditions. Stringent hybridization conditions that can be used to identify nucleic acids within the scope of the invention can include, e.g., hybridization in a buffer comprising 50% formamide, 5×SSC, and 1% SDS at 42° C., or hybridization in a buffer comprising 5×SSC and 1% SDS at 65° C., both with a wash of 0.2×SSC and 0.1% SDS at 65° C. Exemplary stringent hybridization conditions can also include a hybridization in a buffer of 40% formamide, 1 M NaCl, and 1% SDS at 37° C., and a wash in 1×SSC at 45° C. Alternatively, hybridization to filter-bound DNA in 0.5 M NaHPO4, 7% sodium dodecyl sulfate (SDS), 1 mM EDTA at 65° C., and washing in 0.1×SSC/0.1% SDS at 68° C. can be employed. Yet additional stringent hybridization conditions include hybridization at 60° C. or higher and 3×SSC (450 mM sodium chloride/45 mM sodium citrate) or incubation at 42° C. in a solution containing 30% formamide, 1M NaCl, 0.5% sodium sarcosine, 50 mM MES, pH 6.5. Those of ordinary skill will readily recognize that alternative but comparable hybridization and wash conditions can be utilized to provide conditions of similar stringency.

In certain embodiments, the stringency of the wash conditions may affect the degree to which nucleic acids are specifically hybridized to complementary capture agents. Wash conditions used to identify nucleic acids may include, e.g.: a salt concentration of about 0.02 molar at pH 7 and a temperature of at least about 50° C. or about 55° C. to about 60° C.; or, a salt concentration of about 0.15 M NaCl at 72° C. for about 15 minutes; or, a salt concentration of about 0.2×SSC at a temperature of at least about 50° C. or about 55° C. to about 60° C. for about 1 to about 20 minutes; or, multiple washes with a solution with a salt concentration of about 0.1×SSC containing 0.1% SDS at 20 to 50° C. for 1 to 15 minutes; or, equivalent conditions. Stringent conditions for washing can also be, e.g., 0.2×SSC/0.1% SDS at 42° C. In instances wherein the nucleic acid molecules are deoxyoligonucleotides (i.e., oligonucleotides), stringent conditions can include washing in 6×SSC/0.05% sodium pyrophosphate at 37° C. (for 14-base oligos), 48° C. (for 17-base oligos), 55° C. (for 20-base oligos), and 60° C. (for 23-base oligos). See Sambrook, Ausubel, or Tijssen (cited below) for detailed descriptions of equivalent hybridization and wash conditions and for reagents and buffers, e.g., SSC buffers and equivalent reagents and conditions.

A specific example of stringent assay conditions is rotating hybridization at a temperature of about 55° C. to about 70° C. in a salt based hybridization buffer with a total monovalent cation concentration of 1.5M (e.g., as described in U.S. patent application Ser. No. 09/655,482 filed on Sep. 5, 2000, the disclosure of which is herein incorporated by reference) followed by washes of 0.5×SSC and 0.1×SSC at room temperature and 37° C.

Stringent hybridization conditions may also include a “prehybridization” of aqueous phase nucleic acids with complexity-reducing nucleic acids to suppress repetitive sequences. For example, certain stringent hybridization conditions include, prior to any hybridization to surface-bound polynucleotides, hybridization with Cot-1 DNA or with random sequence synthetic oligonucleotides (e.g. 25-mers), or the like.

Stringent assay conditions are hybridization conditions that are at least as stringent as the above representative conditions, where a given set of conditions are considered to be at least as stringent if substantially no additional binding complexes that lack sufficient complementarity to provide for the desired specificity are produced in the given set of conditions as compared to the above specific conditions, where by “substantially no more” is meant less than about 5-fold more, typically less than about 3-fold more. Other stringent hybridization conditions are known in the art and may also be employed, as appropriate.

The term “pre-determined” refers to an element whose identity is known prior to its use. For example, a “pre-determined analyte” is an analyte whose identity is known prior to any binding to a capture agent. An element may be known by name, sequence, molecular weight, its function, or any other attribute or identifier. In some embodiments, the term “analyte of interest”, i.e., a known analyte that is of interest, is used synonymously with the term “pre-determined analyte”.

The term “array” encompasses the term “microarray” and refers to an ordered array of capture agents for binding to aqueous analytes and the like. An “array” includes any two-dimensional or substantially two-dimensional (as well as a three-dimensional) arrangement of spatially addressable regions (i.e., “features”) containing capture agents, particularly polynucleotides, and the like. Any given support may carry one, two, four or more arrays disposed on a surface of a support. Depending upon the use, any or all of the arrays may be the same or different from one another and each may contain multiple spots or features. A typical array may contain one or more, including more than two, more than ten, more than one hundred, more than one thousand, more ten thousand features, or even more than one hundred thousand features, in an area of less than 100 cm², 20 cm² or even less than 10 cm², e.g., less than about 5 cm²,including less than about 1 cm², less than about 1 mm², e.g., 100 μm², or even smaller. For example, features may have widths (that is, diameter, for a round spot) in the range from a 10 μm to 1.0 cm. In other embodiments each feature may have a width in the range of 1.0 μm to 1.0 mm, usually 5.0 μm to 500 μm, and more usually 10 μm to 200 μm. Non-round features may have area ranges equivalent to that of circular features with the foregoing width (diameter) ranges. At least some, or all, of the features are of the same or different compositions (for example, when any repeats of each feature composition are excluded the remaining features may account for at least 5%, 10%, 20%, 50%, 95%, 99% or 100% of the total number of features). Inter-feature areas will typically (but not essentially) be present which do not carry any nucleic acids (or other biopolymer or chemical moiety of a type of which the features are composed). Such inter-feature areas typically will be present where the arrays are formed by processes involving drop deposition of reagents but may not be present when, for example, photolithographic array fabrication processes are used. It will be appreciated though, that the inter-feature areas, when present, could be of various sizes and configurations.

Arrays can be fabricated by depositing (e.g., by contact- or jet-based methods) either precursor units (such as nucleotide or amino acid monomers) or pre-synthesized capture agent. An array is “addressable” when it has multiple regions of different moieties (e.g., different capture agent) such that a region (i.e., a “feature” or “spot” of the array) at a particular predetermined location (i.e., an “address”) on the array will detect a particular sequence. An “array layout” refers to one or more characteristics of the features, such as feature positioning on the support, one or more feature dimensions, and an indication of a moiety at a given location. “Interrogating” the array refers to obtaining information from the array, especially information about analytes binding to the array. “Hybridization assay” references a process of contacting an array with a mobile phase containing analyte. An “array support” refers to an article that supports an addressable collection of capture agents.

“Complementary” references a property of specific binding between polynucleotides based on the sequences of the polynucleotides. As used herein, polynucleotides are complementary if they bind to each other in a hybridization assay under stringent conditions, e.g. if they produce a given or detectable level of signal in a hybridization assay. Portions of polynucleotides are complementary to each other if they follow conventional base-pairing rules, e.g. A pairs with T (or U) and G pairs with C. “Complementary” includes embodiments in which there is an absolute sequence complementarity, and also embodiments in which there is a substantial sequence complementarity. “Absolute sequence complementarity” means that there is 100% sequence complementarity between a first polynucleotide and a second polynucleotide, i.e. there are no insertions, deletions, or substitutions in either of the first and second polynucleotides with respect to the other polynucleotide (over the complementary region). Put another way, every base of the complementary region may be paired with its complementary base, i.e. following normal base-pairing rules. “Substantial sequence complementarity” permits one or more relatively small (less than 10 bases, e.g. less than 5 bases, typically less than 3 bases, more typically a single base) insertions, deletions, or substitutions in the first and/or second polynucleotide (over the complementary region) relative to the other polynucleotide. The region that is complementary between a first polynucleotide and a second polynucleotide (e.g. a target analyte and a capture agent) is typically at least about 10 bases long, more typically at least about 15 bases long, still more typically at least about 20 bases long, or at least about 25 bases long. In various typical embodiments, the region that is complementary between a first polynucleotide and a second polynucleotide (e.g. target analyte and a capture agent) may be up to about 200 bases long, or up to about 120 bases long, up to about 100 bases long, up to about 80 bases long, up to about 60 bases long, or up to about 45 bases long.

“Upstream” as used herein refers to the 5′ direction along a polynucleotide, e.g. an RNA molecule. “Downstream” refers to the 3′ direction along the polynucleotide. Hence, a label downstream of an analyte is located at (or is bound to) a nucleotide moiety that is located in the 3′ direction from the analyte, e.g. bound to the 3′ end of the analyte. Similarly, an “upstream label” references a label that is located at (or is bound to) a nucleotide moiety that is located in the 5′ direction from the analyte, e.g. bound to the 5′ end of the analyte. “3′-” and “5′-” have their conventional meaning as known in the art. A 5′-phosphate is a phosphate group located at the 5′-end of a polynucleotide. A 3′-hydroxyl is a hydroxyl group located at the 3′-end of a polynucleotide. As an example, FIG. 1 illustrates downstream labeling of an analyte. As another example, FIG. 2 illustrates upstream labeling of an analyte. If the polynucleotide is double stranded, one of the strands is selected as the reference strand, e.g. the strand that is labeled, or the strand that is not labeled (or some other criteria or feature of the strand may be used to designate one strand as the reference strand).

Accordingly, in one embodiment of the present invention, a method of labeling RNA in a sample is provided. The method includes contacting the sample with an enzyme having an RNA ligation activity in the presence of a labeled substrate under conditions sufficient to result in coupling of the labeled substrate to the RNA in the sample to provide labeled RNA, the conditions including a DMSO concentration in the range from about 20% to about 30%, wherein the labeled substrate comprises an observable label moiety attached to a nucleotide moiety.

The sample may be any RNA sample, typically a sample containing RNA that has been isolated from a biological source, e.g. any plant, animal, yeast, bacterial, or viral source, or a non-biological source, e.g. chemically synthesized. The dimethylsulfoxide (DMSO) concentration is calculated as volume (e.g. in milliliters) of DMSO divided by total volume (e.g. in milliliters) of the solution containing the DMSO. This quantity is typically cast as a percentage by multiplying by 100%. For example, the DMSO concentration will be in a range of 20% to about 30%, calculated as the volume of DMSO in the solution resulting from contacting the sample with an enzyme having an RNA ligation activity in the presence of a labeled substrate, divided by the total volume of the solution, and then multiplying by 100%. The other components present in the resulting solution will typically be water, buffer components, salt, RNA, labeled substrate, and enzyme having an RNA ligation activity, although other components may also be present. In particular embodiments, the sample includes small RNAs, especially RNAs less than about 500 bases long, e.g. less than about 400 bases long, less than about 300 bases long, less than about 200 bases long, or less than about 100 bases long. In particular embodiments, the sample includes one or more short RNAs, such as e.g. short interfering RNAs (siRNAs), microRNAs (miRNA), tiny non-coding RNAs (tncRNA) and small modulatory RNA (smRNA). See Novina et al., Nature 430: 161-164 (2004). In particular embodiments, the sample includes isolated small RNAs, e.g. the sample results from an isolation protocol for small RNA such as one or more of those listed in this paragraph. In certain embodiments, the small RNA targets may include isolated miRNAs, such as those described in the literature and in the public database accessible via the website located at >>http://www.sanger.ac.uk/cgi-bin/Rfam/mirna/browse.pl<<. In particular embodiments, the sample includes isolated small RNAs, e.g. the sample results from an isolation protocol for small RNA, especially RNAs less than about 500 bases long, e.g. less than about 400 bases long, less than about 300 bases long, less than about 200 bases long, less than about 100 bases long, or less than about 50 bases long.

The enzyme having an RNA ligation activity is typically any RNA ligase enzyme, although other enzymes capable of coupling the labeled substrate to the RNA may be used. In particular embodiments, the enzyme having an RNA ligation activity is capable of coupling a nucleotide (or oligonucleotide, or RNA) having a 5′ phosphate to an oligonucleotide having a 3′ hydroxyl. Exemplary enzymes include T4 RNA ligase available from Amersham/Pharmacia company, ThermoPhage™ RNA ligase II (available from Prokaria LTD, Iceland), or other available RNA ligase enzymes known to be capable of coupling a nucleotide (or oligonucleotide, or RNA) having a 5′ phosphate to an oligonucleotide having a 3′ hydroxyl. In certain embodiments, the enzyme may be selected from yeast poly A polymerase, E. coli poly A polymerase, or terminal transferase (each of which is available from Amersham/Pharmacia). The enzyme having an RNA ligation activity should be selected such that the enzyme is capable performing the coupling when one (or both) of the nucleotide (or oligonucleotide) having a 5′phosphate and/or the oligonucleotide having a 3′ hydroxyl includes a label. Selection of the enzyme having an RNA ligation activity will typically be based on availability of the enzyme and activity of the enzyme under the desired reaction conditions for the coupling (e.g. temperature, pH, ionic strength, source of RNA and/or labeled substrate, structural feature of RNA and/or labeled substrate, concentration of RNA and/or labeled substrate, presence of other materials (e.g. contaminants, salt, surfactant, other solvents) etc.)

The coupling reaction is conducted under conditions sufficient to result in coupling. The conditions of the coupling reaction will generally be selected with regard to the known (previously described) conditions for use of the particular enzyme chosen for use in the methods of the invention, with the specific modifications described herein. As already indicated, the DMSO of the reaction mixture for the coupling reaction will be in the range of 20% to 30%. Other experimental parameters may be selected based on known ranges for the experimental parameters or determined through routine experimentation based on, e.g. efficacy of the labeling reaction. Such other experimental parameters may include, e.g. temperature, pH, ionic strength, source of RNA and/or labeled substrate, structural feature of RNA and/or labeled substrate, concentration of RNA and/or labeled substrate, presence of other materials (e.g. contaminants, salt, surfactant, other solvents) etc.

The labeled substrate comprises an observable label moiety attached to a nucleotide moiety. The nucleotide moiety is typically a mononucleotide moiety or an oligonucleotide moiety. In particular embodiments, the nucleotide moiety is less than about 100 bases long. In embodiments which result in upstream labeling of the RNA (examples shown in FIG. 2), the nucleotide moiety will typically be at least three bases long. In certain embodiments, the nucleotide moiety will be less than 50 bases long, e.g. less than 40 bases long, less than 30 bases long, less than 20 bases long. In some embodiments, the nucleotide moiety will be 1, 2, 3, 4, 5, or 6 bases long. The nucleotide moiety may typically have any desired sequence or even an unknown sequence. In certain embodiments, a plurality of labeled substrates may be used in the same reaction (e.g. a plurality of nucleotide moieties, each having a different sequence, each having an observable label moiety attached), thereby resulting in ligating one of a plurality of nucleotide moieties to each molecule of RNA.

The observable label moiety is a moiety that provides for an observable signal that indicates the presence of the observable label moiety. Typical examples include a chromogenic moiety, a fluorophore, a mass label, a spin label, a radiolabel, or other labels known in the art. In particular embodiments, the observable label moiety is a fluorophore selected from the group consisting of Cy3, Cy5, and an Alexa dye. Further examples of label moieties include any commercially available fluorophores that can be conjugated to mononucleotides or polynucleotides, e.g. dyes from Molecular Probes (Eugene, Oreg. and Leiden, The Netherlands) such as the Alexa Fluor series (example: Alexa 350, Alexa 430, Alexa 532, Alexa 546, Alexa 568, and Alexa 594) and the series of BODIPY conjugates. Other examples include: Tamra, Fluorescein, carboxyfluorescene, JOE, rhodamine, carboxyrhodamine, CY series, Oyster series. More information about commercially available dyes for oligonucleotide conjugation can be found at the website located at >>http://www.synthegen.com<<. Any such dyes may potentially be used in accordance with the methods described herein. Although the examples described herein use a fluorophore as the label, it will be apparent to those of ordinary skill that other labels may be used (instead of a fluorophore, or even in addition to a fluorophore). Such labels typically are well known in the art.

The observable label moiety may be attached to the nucleotide moiety at any site of the nucleotide moiety that is compatible with the ligation reaction. In other words, the label moiety should not prevent the ligation reaction, e.g. by interfering with the enzyme having an RNA ligation activity. In embodiments of downstream labeling (such as illustrated in FIG. 1), the label moiety may be attached to the nucleotide moiety via the 3′-site at the end of the nucleotide moiety or may be attached at any available “internal” site (i.e. other than the 3′-site or 5′-site) of the nucleotide moiety. In embodiments of upstream labeling (such as illustrated in FIG. 2), the label moiety may be attached to the nucleotide moiety via the 5′-site at the end of the nucleotide moiety or may be attached at any available “internal” site (i.e. other than the 3′-site or 5′-site) of the nucleotide moiety. The observable label moiety could be incorporated as a special phosphoramidite during the oligoribonucleotide synthesis or as a post-synthetic modification. An example of the phosphoramidite method includes direct coupling of label-containing phosphoramidite during synthesis of the oligonucleotide moiety, or the incorporation of amino-activated phosphoramidite during synthesis of the oligonucleotide moiety, which enables post-synthetic coupling to desired observable label moiety. In particular embodiments, the observable label moiety is attached to the nucleotide moiety via a linking group, wherein the linking group may be attached to the nucleotide moiety via a 5′ terminus or a 3′ terminus of the nucleotide moiety or any other available site of the nucleotide moiety. The linking group may be any linking group known in the art that does not prevent the ligation reaction (e.g. does not prevent the enzyme having an RNA ligation activity from ligating the RNA and the labeled substrate). Any such linking groups or other means of attachment of the label moiety to the nucleotide moiety known in the art may provide for the labeled substrate.

Thus, in particular embodiments, the labeled substrate has the structure: N-D

wherein: N is selected from a mononucleotide moiety or an oligonucleotide moiety having a length of less than about 100 bases, and

-   -   D is an observable label moiety (e.g. attached to N via a 5′         terminus or a 3′ terminus of N, or any other available site of         N).

An embodiment of a method in accordance with the present invention is illustrated in FIG. 1. In FIG. 1, the method 100 of labeling RNA includes adding 104 DMSO 106 to the sample (which includes the RNA 102). The labeled substrate 110A, 110B or 110C may then be added 108 to the resulting solution (containing the DMSO 106 and the RNA 102 from the sample). The labeled substrate 110A, 110B or 110C typically is a mononucleotide with a 3′ fluorophore and 5′ phosphate 110A, an oligonucleotide with a 5′ phosphate and one or more internal fluorophore(s) 110B, or an oligonucleotide with a 3′ fluorophore and 5′ phosphate 110C. (In an embodiment in which the labeled substrate 110B has one or more internal fluorophores, the 3′ end of the oligonucleotide lacks the 3′ hydroxyl, as indicated by the ‘X’ in FIG. 1. For example, a periodate oxidation reaction may be used to modify the 3′ end, or the 3′ end may be modified by having a fluorophore bound thereto.) The enzyme having an RNA ligation activity 114 is also added 112. In typical embodiments, the concentrations of the solutions and the volumes added are planned to provide that the resulting solution has the desired concentration of DMSO (e.g. in the range of about 20% to about 30%, more typically in the range of about 22% to about 28%, even more typically in the range of about 24% to about 26%). The resulting solution is then allowed to react 116 under conditions and for a time sufficient for the coupling of the labeled substrate to the RNA to occur, thereby providing the labeled RNA 118A, 118B, or 118C. Typical conditions 120 of overnight incubation at 16° C. are shown for the embodiment of FIG. 1, although these conditions may vary depending on the particular enzyme used and the RNA and labeled substrate provided. In the illustrated embodiment, the label is a fluorophore 111, but other labels may be used as long as the coupling of the labeled substrate to the RNA may still occur. Selection and optimization of the conditions is within routine experimentation for one of ordinary skill in the art given the disclosure herein.

Another embodiment of a method in accordance with the present invention is illustrated in FIG. 2. In FIG. 2, the method 200 of labeling RNA includes pre-treating the RNA 102 by performing a periodate oxidation reaction 222 on the initial RNA sample. The periodate oxidation reaction helps prevent the RNA from concatenating or from ligating to itself when the enzyme having an RNA ligation activity is later added. The periodate oxidation reaction 222 is followed with inactivation and de-salting 224 of the solution resulting from the periodate oxidation reaction. The periodate oxidation reaction may be performed using any protocol known in the art. See, e.g. Neu, Harold C., et al., J. Biol. Chem 239: 2927-34 (1964); Kurata, Shinya, et al., Nucleic Acids Res. 31: e145 (2003); England, T. E., et al., Meth. Enzymol. 65: 65-74 (1980); England, T. E., et al., Nature 275: 560-61 (1978); and Maruyama, K., et al., Gene 138: 171-74 (1994). In a typical periodate oxidation reaction, ribonucleotides are treated with estimated 5-fold molar excess of sodium metaperiodate for 20 minutes to 1 hour at 0° C. to room temperature in the dark. The ribonucleotides can be cleaned from the solution by ethanol precipitation or desalting columns, such as BioRad Micro Bio-Spin™ 6.

Continuing with the embodiment shown in FIG. 2, the method 200 includes adding 204 DMSO 206 to the solution containing the RNA 202. The labeled substrate 210A or 210B may then be added 208 to the resulting solution (containing the DMSO 206 and the RNA 202 from the sample). The labeled substrate 210A or 210B typically is an oligonucleotide with a 5′ fluorophore 210A or an oligonucleotide with one or more internal fluorophore(s) 210B. The enzyme having an RNA ligation activity 214 is also added 212. In typical embodiments, the concentrations of the solutions and the volumes added are planned to provide that the resulting solution has the desired concentration of DMSO (e.g. in the range of about 20% to about 30%, more typically in the range of about 22% to about 28%, even more typically in the range of about 24% to about 26%). The resulting solution is then allowed to react 216 under conditions and for a time sufficient for the coupling of the labeled substrate to the RNA to occur, thereby providing the labeled RNA 218A or 218B (products corresponding to 210A or 210B, respectively). Typical conditions 220 of overnight incubation at 16° C. are shown for the embodiment of FIG. 2, although these conditions may vary depending on the particular enzyme used and the RNA and labeled substrate provided. For example, the temperature of the incubation will typically be in the range from about 4° C. to about 37° C., more typically in the range from about 10° C. to about 30° C., still more typically in the range from about 14° C. to about 20° C. In the illustrated embodiment, the label is a fluorophore 211, but other labels may be used as long as the coupling of the labeled substrate to the RNA may still occur. Selection and optimization of the conditions is within routine experimentation for one of ordinary skill in the art given the disclosure herein.

In the embodiment illustrated in FIG. 1, the labeled substrate denominated 110A is shown as a mononucleotide attached to a fluorophore. In other embodiments illustrated in FIG. 1, the labeled substrates 110B and 110C are shown as an oligonucleotide having one or more fluorophore(s) attached to the oligonucleotide, in which the oligonucleotide does not have a 3′ hydroxyl. The lack of the 3′ hydroxyl helps prevent concatenation of the labeled substrate. This is analogous to the embodiment shown in FIG. 2, wherein the RNA 202 is subjected to periodate oxidation to help prevent the RNA from concatenating or from ligating to itself. The labeled substrate 110C is shown as an oligonucleotide having a fluorophore(s) attached to an oligonucleotide at the 3′ end of the oligonucleotide. In FIG. 2, the labeled substrate denominated 210A and 210B illustrate representative embodiments in which the labeled substrate includes an oligonucleotide attached to one or more fluorophores. In certain embodiments, the labeled substrate includes an oligonucleotide having attached thereto a plurality, e.g. 2, 3, 4, 5, or more label moieties, up to about 10, 20, 30 or more label moieties, attached to the oligonucleotide moiety at internal (not terminal) sites. In certain such embodiments, a label moiety is also attached to the oligonucleotide via one of the 3′ or 5′ terminal carbons of the oligonucleotide.

In particular embodiments, the enzyme having an RNA ligation activity catalyzes a coupling reaction between a donor molecule having a 5′-phosphate and an acceptor molecule having a 3′-hydroxyl, as shown in the reaction:

Where: Acc-3′-OH is the acceptor molecule having a 3′-hydroxyl;

-   -   PO₄-5′-Don is the donor molecule having a 5′-phosphate;     -   Acc-3′-O—PO₃-5′-Don is the product having the coupled donor and         acceptor moieties (e.g. the labeled RNA); and     -   (enz) is the enzyme having an RNA ligation activity.

In certain embodiments, such as that illustrated in FIG. 1, the acceptor molecule is the RNA 102 and the donor molecule is the labeled substrate 110A, 110B or 110C. The resulting product 118A, 118B, or 118C has the labeled substrate moiety downstream from the RNA, i.e. the product is a downstream labeled RNA. In other embodiments, such as that illustrated in FIG. 2, the acceptor molecule is the labeled substrate 210A or 210B and the donor molecule is the RNA 202. The resulting product 218A or 218B has the labeled substrate moiety upstream from the RNA, i.e. the product is an upstream labeled RNA. Thus, based upon selection of the RNA and labeled substrate as disclosed herein, methods in accordance with the present invention may result in either upstream or downstream labeled RNA.

It should be noted that the general utility of the method is not limited to the particular sequence of steps shown in the figures. Other sequences of steps leading to essentially similar results are intended to be included in the invention. For example, in certain embodiments, the labeled substrate may be dissolved in a solution that includes the DMSO, and the resulting solution mixed with the sample prior to contacting with the enzyme having an RNA ligase activity. Thus, in particular embodiments, the invention includes any process which results in contacting the sample with the enzyme having an RNA ligation activity in the presence of the labeled substrate under conditions which include a DMSO concentration in the range from about 20% to about 30%.

With reference to FIG. 1, in certain embodiments, after the DMSO 106 is added 104 and before the enzyme having an RNA ligation activity 114 is added 112, the method 100 includes heating the solution containing the DMSO 106 and the RNA 102 from the sample. In this optional heating step, the RNA is typically heated to a temperature of at least about 80° C. (e.g. at least about 85° C., at least about 90° C., at least about 95° C.; and up to about 105° C. or 110° C.) under conditions that include a DMSO concentration of at least about 40% DMSO (typically up to about 60% DMSO, although in some embodiments the DMSO concentration may be up to 70% DMSO, up to 80% DMSO, or even more). This optional heating is maintained for at least 10 seconds, typically at least about 20 seconds, at least about 30 seconds, at least about 1 minute, at least about 2 minutes, and up to about 15 minutes, or more. In particular embodiments, reaction solutions of up to about 50 microliters are heated for about 30 to about 60 seconds per 5-10 microliters of reaction solution. After the heating, the RNA is typically quickly cooled (e.g. to less than about 40° C., more typically less than about 20° C., or in some embodiments less than about 5° C.) before adding the enzyme having an RNA ligation activity. With reference to FIG. 2, a similar heating step may optionally be included in embodiments such as that illustrated in the figure.

It should be noted that, in particular embodiments, the RNA in the sample is isolated via a process that results in the RNA in the sample having a 5′-phosphate. For embodiments such as that pictured in FIG. 1, in which the RNA in the sample does not have a 5′-phosphate, a preparatory treatment of subjecting the RNA in the sample to a dephosphorylation reaction is conducted prior to labeling the RNA in the sample by the ligation method illustrated in FIG. 1. Such dephosphorylation reactions are well known in the art, for example, treating the RNA sample with an enzyme having a 5′-phosphatase activity, e.g. calf intestine alkaline phosphatase, shrimp alkaline phosphatase, or E. coli alkaline phosphatase, or any other method of dephosphorylating the RNA known in the art. Thus, in certain embodiments, the method of labeling RNA in a sample includes, prior to contacting the sample with the enzyme having an RNA ligation activity, contacting the sample with an enzyme having a 5′-phosphatase activity to remove 5′-phosphate groups from the RNA in the sample.

In some embodiments, the labeled substrate has only one observable label moiety attached to the nucleotide moiety. In such embodiments, the labeled RNA will consist essentially of RNA labeled with a single label moiety (i.e. each labeled RNA molecule will have only one observable label moiety attached—referenced herein as “singly-labeled RNA”). This potentially provides increased ease of use in quantitative methods using the labeled RNA.

In other embodiments, the nucleotide moiety of the labeled substrate has a plurality of observable label moieties. In such embodiments, when the labeling reaction is performed to yield the labeled RNA, each labeled RNA molecule will have a plurality of observable label moieties (referenced herein as “multiply-labeled RNA”). Thus, the labeled RNA will consist essentially of RNA labeled with a plurality of label moieties. This increased labeling of the RNA may provide for greater sensitivity in analyses using the labeled RNA. In particular embodiments, each labeled RNA molecule in the sample will be labeled with a consistent number of observable label moieties (relative to the other labeled RNA molecules in the sample). This provides opportunity for more quantitative analysis of labeled RNA than in methods that provide an inconsistent number of observable labels per labeled RNA molecule.

In certain embodiments, methods of performing an array analysis of an RNA sample are provided. In certain embodiments, the invention provides a method of performing an array analysis wherein the method includes labeling the RNA in the sample to provide labeled RNA using a labeling method in accordance with the methods described herein. The labeled RNA is then contacted with an array under conditions sufficient to provide for specific binding of labeled RNA to the array. The array typically is then interrogated to provide data on binding of the labeled RNA to the array.

Standard hybridization techniques (using stringent hybridization conditions) are used to hybridize a labeled sample to a nucleic acid array. Suitable methods are described in references describing CGH techniques (Kallioniemi et al., Science 258:818-821 (1992) and WO 93/18186). Several guides to general techniques are available, e.g., Tijssen, Hybridization with Nucleic Acid Probes, Parts I and II (Elsevier, Amsterdam 1993). For descriptions of techniques suitable for in situ hybridizations, see Gall et al. Meth. Enzymol., 21:470-480 (1981); and Angerer et al. in Genetic Engineering: Principles and Methods (Setlow and Hollaender, Eds.) Vol 7, pgs 43-65 (Plenum Press, New York 1985). See also U.S. Pat. Nos. 6,335,167; 6,197,501; 5,830,645; and 5,665,549; the disclosures of which are herein incorporated by reference. Hybridizing the sample to the array is typically performed under stringent hybridization conditions, as described herein and as known in the art. Selection of appropriate conditions, including temperature, salt concentration, polynucleotide concentration, time(duration) of hybridization, stringency of washing conditions, and the like will depend on experimental design, including source of sample, identity of capture agents, degree of complementarity expected, etc., and are within routine experimentation for those of ordinary skill in the art to which the invention applies.

Following hybridization, the array-surface bound polynucleotides are typically washed to remove unbound and not tightly bound labeled nucleic acids. Washing may be performed using any convenient washing protocol, where the washing conditions are typically stringent, as described above.

Following hybridization and washing, as described above, the hybridization of the labeled target nucleic acids to the capture agents is then detected using standard techniques of reading the array, i.e. the array is interrogated. Reading the resultant hybridized array may be accomplished by illuminating the array and reading the location and intensity of resulting fluorescence at each feature of the array to detect any binding complexes on the surface of the array. For example, a scanner may be used for this purpose, which is similar to the AGILENT MICROARRAY SCANNER available from Agilent Technologies, Palo Alto, Calif. Other suitable devices and methods are described in U.S. patent applications: Ser. No. 09/846,125 “Reading Multi-Featured Arrays” by Dorsel et al.; and U.S. Pat. No. 6,406,849. However, arrays may be read by any other method or apparatus than the foregoing, with other reading methods including other optical techniques (for example, detecting chemiluminescent or electroluminescent labels) or electrical techniques (where each feature is provided with an electrode to detect hybridization at that feature in a manner disclosed in U.S. Pat. No. 6,221,583 and elsewhere). In the case of indirect labeling, subsequent treatment of the array with the appropriate reagents may be employed to enable reading of the array. Some methods of detection, such as surface plasmon resonance, do not require any labeling of nucleic acids, and are suitable for some embodiments.

Results from the reading or evaluating may be raw results (such as fluorescence intensity readings for each feature in one or more color channels) or may be processed results (such as those obtained by subtracting a background measurement, or by rejecting a reading for a feature which is below a predetermined threshold, normalizing the results, and/or forming conclusions based on the pattern read from the array (such as whether or not a particular target sequence may have been present in the sample, or whether or not a pattern indicates a particular condition of an organism from which the sample came).

In certain embodiments, results from interrogating the array are used to assess the level of binding of the population of labeled nucleic acids to capture agents on the array. The term “level of binding” means any assessment of binding (e.g. a quantitative or qualitative, relative or absolute assessment) usually done, as is known in the art, by detecting signal (i.e., pixel brightness) from a label associated with the sample nucleic acids, e.g. the digested sample is labeled. The level of binding of labeled nucleic acid to capture agent is typically obtained by measuring the surface density of the bound label (or of a signal resulting from the label).

In certain embodiments, a surface-bound polynucleotide may be assessed by evaluating its binding to two populations of nucleic acids that are distinguishably labeled. In these embodiments, for a single surface-bound polynucleotide of interest, the results obtained from hybridization with a first population of labeled nucleic acids may be compared to results obtained from hybridization with the second population of nucleic acids, usually after normalization of the data. The results may be expressed using any convenient means, e.g., as a number or numerical ratio, etc.

EXAMPLES

The practice of the present invention will employ, unless otherwise indicated, conventional techniques of synthetic organic chemistry, biochemistry, molecular biology, and the like, which are within the skill of the art. Such techniques are explained fully in the literature. Unless otherwise defined herein, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the invention belongs.

The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to perform the methods and use the compositions disclosed and claimed herein. Efforts have been made to ensure accuracy with respect to numbers (e.g., amounts, temperature, etc.) but some errors and deviations should be accounted for. Unless indicated otherwise, parts are parts by weight, temperature is in ° C. and pressure is at or near atmospheric. Standard temperature and pressure are defined as 20° C. and 1 atmosphere.

Experimental Methods:

RNA ligation was assessed with synthetic RNA oligonucleotides (21-23 nucleotides, Dharmacon) in reaction solutions containing 0, 15, 20, 25, and 30% DMSO. The reactions containing 25% DMSO were assayed with and without the pre-heating step. Stock solutions of 20 μM RNA oligonucleotides were stored in 1×TE (10 mM Tris-HCl, pH 7.5, 1 mM EDTA). Initial mixtures of RNA, DMSO and water were first assembled. For pre-heated samples, the heated mixture contained 40-70% DMSO and were heated using a 104° C. heating block for 1.5-2 minutes. The heated samples were immediately set on ice for >5 minutes prior to final assembly. The final reaction contains 1×Amersham Pharmacia RNA ligase buffer (50 mM Tris-HCl, pH 7.5, 10 mM MgCl₂, 10 mM DTT, 1 mM ATP, 60 ng/μL BSA) 1 unit/μL T4 RNA ligase, 100 μM, 5′-phosphate-cytidyl-phosphate-Cy5-3′ (pCpCy5) or 5′-phosphate-cytidyl-phosphate-Cy3-3′(pCpCy3) (Dharmacon) and 2-4 μM RNA oligonucleotides. The reactions were incubated at 16° C. overnight. RNA ligase was inactivated by heating the reaction solutions using a 104° C. heating block for 1.5-2 minutes, followed by immediately setting on ice for >5 minutes.

The labeling efficiency was determined by 5′-phosphorylation of RNA ligation reaction aliquots with radioactive P³²-gamma-ATP. The resulting mixture was desalted with Micro Bio-Spin™ (BioRad) desalting columns. The desalted mixture was loaded onto denaturing polyacrylamide gel. Since the ligation products contain an extra nucleotide and fluorophore, they have a lower electrophoretic migration rate than the unligated precursors. P³²-labeled RNA bands are visualized and quantified with phosphorimager (Molecular Dynamics). The ligation efficiency was determined by the ratio of ligated vs. unligated P³²-labeled RNA bands. Thus, ligation efficiency may be expressed as the mol % of initial RNA that winds up having an attached label moiety.

Description of Experiments:

In the experiments described here, T4 RNA ligase is used to label synthetic RNA oligonucleotides with 5′-phosphate-cytidyl-phosphate-Cy5-3′ (pCpCy5) or 5′-phosphate-cytidyl-phosphate-Cy3-3′(pCpCy3). The reaction conditions described here have been observed to result in ligation efficiencies of about 60% or more, e.g. about 70% or more, or 80% or more, up to about 95% or more, e.g. up to about 99% with minimal sequence discrimination. This was accomplished by reacting at 25% DMSO, 16° C. overnight, with donor to acceptor ratio of >12.5:1. The reaction buffer contains 50 mM Tris-HCl, pH 7.5, 10 mM MgCl2, 10 mM DTT, 1 mM ATP, 60 μg/mL BSA, and 25% DMSO. Typical reactions are 10 μL with 2 μM RNA, 100 μM pCpCy5 or pCpCy3, and 4 units T4 RNA ligase (Amersham/Pharmacia). Reaction efficiency seemed unaffected by increasing the RNA concentration to 8 μM or decreasing enzyme to 2 units.

The labeling efficiency was determined by first performing the ligation reaction. An aliquot of the ligation mixture was then labeled on the 5′ end with radioactive P³²-γ-ATP and T4 Polynucleotide Kinase. The control sample, which did not undergo ligation reaction, and final reaction mixture was denatured with formamide and assessed with denaturing polyacrylamide gel electrophoresis (1× or 0.5×TBE, 50% urea, 15-20% polyacrylamide with 19:1 acrylamide to bisacrylamide ratio, at about 50° C.). The resulting gel was scanned with Molecular Dynamics Storm Phosphorimager for pCpCy5-labeled RNA. The ligation product was clearly visible as a red fluorescent miRNA. The gel was then exposed to phosphor screen to determine the pCpCy5-labeling efficiency. Since the addition of pCpCy5 increases the acceptor miRNA by 1 nucleotide and a fluorophore, the mobility of the cy5-labeled strand was lower than the unreacted strand. They appear as distinct bands when scanned in the phosphor mode on the phosphoimager; this was further verified by the relative mobility between the ligase reacted samples and unreacted controls (both the ligase reacted samples and the unreacted controls were labeled with P³²-γ-ATP). Thus the relative level of radioactivity between the Cy-labeled and unlabeled bands reveals the ligation efficiency. The reaction efficiency of pCpCy3 was determined similarly except the Cy3 labeled strand was undetectable by the fluorescent mode of the phosphorimager. The product and reactant miRNA bands of the Cy3 reaction were defined by the mobility of Cy5 reactions in polyacrylamide gel electrophoreseis and phosphorimager analysis.

Ligation efficiency under different reaction conditions was extensively tested with pCpCy5 and 4 separate synthetic oligonucleotides (SEQ ID NOs:1-4), each of which contains the same sequence as drosophila miRNA (as indicated):

TABLE 1 Test Sample miRNAs SEQ 3′ ID Terminal # NO: Name Sequence NT NT 1 dme-miR-3 UCACUGGGCAAAGUGUGUCUCA A 22 2 dme-let-7 UGAGGUAGUAGGUUGUAUAGU U 21 3 dme-miR-14 UCAGUCUUUUUCUCUCUCCUA A 21 4 dme-miR-31a UGGCAAGAUGUCGGCAUAGCUGA A 23 These miRNAs were labeled with 80-99% efficiency when the reaction mixture contained 95% (molar ratio) competitors composing of other miRNAs and longer single stranded RNAs (100-500 nts). Thus it is reasonable to expect high labeling efficiency in heterogeneous biological RNA mixtures.

After optimization of labeling efficiencies of these RNAs with Cy5, the labeling reaction was expanded to include the following sequences (SEQ ID NOs:6-10) with pCpCy5 and pCpCy3 in separate studies. These additional strands address any bias that may result from 3′ terminal nucleotide, potential secondary structures and nucleotide content of the miRNA.

TABLE 2 Additional Test Sample miRNAs 3′ SEQ Termi- ID nal # NO: Name Sequence NT NT 5 dme-miR-2b UAUCACAGCCAGCUUUGAGGAGC C 23 6 dme-miR-6 UAUCACAGUGGCUGUUCUUUUU U 22 7 dme-miR-184* CCUUAUCAUUCUCUCGCCCCG G 21 8 dme-miR-285 UAGCACCAUUCGAAAUCAGUGC C 22 9 dme-miR-308 AAUCACAGGAUUAUACUGUGAG G 22 10 dme-miR-316 UGUCUUUUUCCGCUUACUGGCG G 22

Potentially, the RNA ligase method can be used for dyes other than Cy5 and Cy3, but the efficiency may differ from the ones presented here. Moreover, it is possible to determine the labeling efficiencies of each individual miRNA of a given set and perform highly quantitative microarray experiments by correlating fluorophore counts with number of molecule. For example, in an array hybridization experiment wherein an array is contacted with a labeled RNA sample, it is possible to ascertain the total quantity of fluorophores in a given area of the array by interrogating (or scanning) the array; given the labeling efficiency of the labeled RNA sample (determined as disclosed herein), the quantity of RNA hybridized to the given area of the array may be determined.

Given that the approximate labeling efficiency may be determined (as described herein), in particular embodiments the present invention thus provides quantitative methods of performing array hybridization experiments. It is expected that this will provide a more sensitive assay system for the detection of variations of miRNA, such as found in developmental stages, tissue samples, disease states, as well as any individual and/or abnormal variations. Moreover, if more viral miRNAs are identified, this can become a novel diagnostic tool for active as well as latent viral infections

Determination of Labeling Efficiency of miRNAs in Complex Samples

RNA ligase is used to label a complex RNA mixture, such as the total RNA or isolated mixtures of small RNAs from biological samples. The labeled mixtures are run on denaturing polyacrylamide gel and Northern blots are performed of individual miRNAs with radioactive probes. The RNAs labeled by RNA ligase will have a lower mobility relative to its unlabeled counterpart. Thus each target sequence will run as a doublet when probed by Northern blot. The ratio of RNA species in these doublets reflects the molar ratio of the RNA ligase labeled vs. unlabeled RNA species.

Microarray Hybridization:

The synthetic miRNA set forth above were either labeled with Cy5 or Cy3 and hybridized onto microarrays as follows:

Labeled miRNA were desalted with BioRad Micro Bio-Spin™ 6 (as directed by BioRad instructions) to remove free fluorescent tags. The desalted miRNA was added to solution containing water and carrier (25-mer DNA with random sequence). The solution was heated for approximately 1 minute per 10 ul solution at 100° C. and immediately placed on ice. After cooling, 2× Agilent Hyb Buffer (1225 mM LiCl, 300 mM Li-MES, pH 6.1, 12 mM EDTA, 3.0% (w/v) lithium dodecyl sulfate, 2.0% (w/v) Triton X-100) was added to the mixture and the viscous liquid was mixed carefully. The final solution contained 1× Hyb Buffer and 0.1 μg/μl random 25-mer. The concentration of miRNA was varied for different experiments.

Hybridization was performed with SureHyb hybridization chamber (Agilent Part Number: G2534A) and place on rotisserie of hybridization oven overnight. The hybridization temperature was tested at 50° C. and 60° C.

After hybridization was completed, the Sure-Hyb chamber complex was removed from the oven and immediately disassembled in Wash Buffer 1 (6×SSC, 0.005% Triton X-102) at room temperature. The microarray was transferred to a fresh wash chamber containing Wash Buffer 1 and washed by stirring for 10 minutes at room temperature. The microarray was then washed in Wash Buffer 2 (0.1×SSC, 0.005% Triton X-102) by stirring at room temperature for 5 minutes. The microarray was slowly lifted out of the wash chamber after washing and dried with nitrogen as needed. The microarrays were scanned with Agilent Scanner (Agilent Product Number: G2565BA). The scanned data was extracted with Agilent Feature Extraction Software (Agilent Product Number; G2567AA) and the green and red background-subtracted signals were evaluated for hybridization efficiency and specificity. Data was further analyzed using Spotfire software and Microsoft Excel.

While the foregoing embodiments of the invention have been set forth in considerable detail for the purpose of making a complete disclosure of the invention, it will be apparent to those of skill in the art that numerous changes may be made in such details without departing from the spirit and the principles of the invention. Accordingly, the invention should be limited only by the following claims.

All patents, patent applications, and publications mentioned herein are hereby incorporated by reference in their entireties, provided that, if there is a conflict in definitions, the definitions provided herein shall control. 

1. A method of sample analysis comprising: a) heating a sample comprising a plurality of miRNAs to at least 80° C. under conditions including at least 40% DMSO; b) contacting said sample with RNA ligase in the presence of a labeled substrate under conditions sufficient to result in coupling of the labeled substrate to the miRNAs in the sample to provide labeled miRNAs, the conditions including a DMSO concentration in the range from about 20% to about 30%, wherein the labeled substrate comprises an observable label moiety and a nucleotide moiety, the nucleotide moiety having a 5′ phosphate and a 3′ terminus, the observable label moiety directly attached to the 3′ terminus, wherein the observable label moiety is selected from a chromogenic moiety or a fluorophore; c) contacting said labeled miRNAs to an array of probes under conditions suitable for said labeled miRNA to hybridize to said probes; and d) interrogating said array to provide quantitative data on binding of said labeled miRNAs to said array.
 2. the method of claim 1, wherein the nucleotide moiety is selected from a mononucleotide moiety or an oligonucleotide moiety.
 3. the method of claim 1, wherein the nucleotide moiety has a length in the range from 2 to about 50 bases.
 4. The method of claim 1, wherein the miRNAs in the sample comprises isolated miRNAs having length less than 500 bases.
 5. The method of claim 1, wherein the miRNAs in the sample comprises isolated miRNAs having length less than 200 bases.
 6. The method of claim 1, wherein the observable label moiety is a fluorophore selected from the group consisting of Cy3, Cy5, or an Alexa dye.
 7. The method of claim 1, wherein the nucleotide moiety has a free 5′-phosphate and a 3′ terminus bound to the observable label moiety.
 8. The method of claim 1, wherein the labeled substrate comprises a plurality of observable label moieties and the labeled miRNAs consists essentially of RNA labeled with a plurality of label moieties.
 9. The method of claim 1, wherein the labeled substrate comprises a single observable moiety and the labeled miRNAs consists essentially of singly-labeled miRNAs.
 10. The method of claim 1, wherein the labeled miRNAs is at least 70% of the initial miRNAs in the sample.
 11. The method of claim 1, wherein the duration of heating is at least 10 seconds.
 12. The method of claim 1, further comprising, prior to contacting the sample with the enzyme having an RNA ligation activity, contacting the sample with an enzyme having a 5′-phosphatase activity to remove 5′-phosphate groups from the miRNAs in the sample.
 13. The method of claim 1, wherein the enzyme is capable of coupling a species selected from the group consisting of a nucleotide having a 5′ phosphate, an oligonucleotide having a 5′ phosphate, and an RNA having a 5′ phosphate to an oligonucleotide having a 3′ hydroxyl. 